1. Field of the Invention
The invention pertains to sonic transducer devices employing surface acoustic waves adapted for use in the measurement of strain, displacement, acceleration, force or related parameters and more particularly relates to transducers utilizing relative changes of the velocity of propagation of acoustic surface waves and of the effective length of flexible elements constituted of materials, for example, of the piezoelectric or ferroelectric kinds. 2. Description of the Prior Art
Generally, prior art accelerometer and other displacement or strain sensing device encompass variety of structures and principles. Each approach has been found to have its particular merits; but many defects are also present, such as lack of sensitivity and reliability on the one hand, and fragility and high cost on the other. Some accelerometer arrangements, for example, require expensive auxiliary equipment, such as feed back mechanisms for providing reliable calibration or constant temperature enclosures. Many such prior art arrangements are inherently analog in nature and do not lend themselves directly to use in digital equipment.
The prior art includes a variety of piezoresistor transducers, including direct stress transducers of the kind employing a flexible cantilever clamped at one end to which acceleration or physically applied forces are directed at the unclamped end for flexing the cantilever. In these transducers, the basic structural member (the cantilever) often does not contribute greatly to the sensitivity of the apparatus, merely transmitting the stress to be measured to a more sensitive sensor element such as a piezoresistor affixed to a flexing surface of the cantilever. Measurement of current flow through such resistors yields a measure of strain of the cantilever.
Such devices are, however, expensive and difficult to manufacture and strict controls must be exercised to yield a uniform product. In particular, reliable bonding of the strain gauge to the cantilever is difficult. Since resistivity of an exposed resistor element is to be measured by measuring flow of electrical current through it, the devices are sensitive to changing ambient temperature and humidity conditions. Strict maintenance of calibration requires many special precautions. Furthermore, the devices are inherently analog in nature and are best suited for application when the associated display or other utilization device is an analog device. Such measurement devices generally demonstrate troublesome zero drift and scale factor drift with variations in temperature and in power supply voltage and may therefore demonstrate significant warm up drifts and even continued instability after the initial operating period.